Setipiprant is an orally available, selective CRTH2 antagonist (IC50: 6.0 nM). CRTH2 is a G protein-coupled receptor for prostaglandin (PGD2). PGD2 is produced by the mast cells and is a key mediator in various inflammatory diseases, including allergy and asthma. Binding of PGD2 to CRTH2, which are expressed on the surface of blood-borne cells, induces chemotaxis of Th2 cells, basophils, and eosinophils, and stimulates cytokine release from these cells.